Instructions:

Bring a pot of water to rapid boil, add pasta and reduce to a gentle simmer. Cook until just on the firm side of al dente, about 6-8 minutes. (Shells will soften during baking).

Melt butter in a large saucepan. When melted, add flour and blend to form a roux. When roux smells slightly nutty, add 2 cups milk and whisk to combine. When sauce begins to thicken, season with salt and mustard. Taste and adjust.